Multiple tones are likely to be used in the telephones systems. Firstly, there are the various control and call status information which are commonly known as call progress tones which are typically transmitted by a central office and transmitted to a calling party in order to keep the latter informed of the current status of the call.
Secondly, the call may also be completed with the use of the Dual Tone, Multi-Frequency (DTMF) tones, particularly in use in advanced facsimile systems or also more sophisticated voice-server applications which rapidly develop, as in mail-order business or banking applications DTMF signal is a digital signal, being one of sixteen combinations of four frequencies from a high-frequency group and four frequencies from a low-frequency group, which is issued when a push button is depressed on the telephone and familiarly called a push tone or a dial tone.
FIG. 1 illustrates the general architecture of a voice-server application based on a voice-server system 101 which can be accessed by telephones 103 located anywhere in the country through the telephone network.
In addition to the necessity of handling the numerous call-progress tones which are likely to be found on the telephone line, the modern tone-detection systems must have a high level of discrimination since each of the multiples tones which are likely to be found on the network, e.g. the dial-tone, ring-back tone, busy tone, alerting tones, reorder tone, etc. . . has a different frequency value in each country.
Considering the common dial-tone signal: while the dial-tone signal has a frequency value of 440 Hz in France, the same consists of a 425 Hz tone having a specific cadence in Italy, and is a continuous tone made by combining signals having frequencies of 350 Hz and 440 Hz in the U.S.
Various digital tone detection systems exist in the art. Some are based on a sophisticated Fast Fourier Transformation (FFT), while others use digital band-pass filters which arc adjusted .at the desired frequencies to be detected. The former systems necessitate a great deal of digital processing resources and the latter can not easily adjusted to match the wide range of tones which exists in the different countries.
Therefore, there is a need in the industry for a simple call-progress method which can match with characteristics of the-numerous telephone networks existing in the different countries.